The invention relates to a printing machine with a printing cylinder, the rotation of which can be driven, and a cleaning device for the local cleaning of blocks, clamped on the printing cylinder, for which the cleaning device is constructed as a rotatable cleaning roller, which can be placed in contact with the peripheral surface of the printing cylinder and is equipped with cleaning organs, so that the angular positions of the cleaning organs vary in the axial direction of the cleaning roller.
For a printing machine, especially for a flexographic printing machine, there is increasing contamination of the blocks during the printing operation due to dust on the material being printed, due to the dust in the surrounding air and due to drying residues of ink. The dirt is deposited especially on the edges of the raised, printing parts of the blocks. Especially when a finely structured pattern, such as letters in a very small font or halftone images with a fine grid of points, is being printed, the lower lying, not printing parts of the blocks can easily become blocked as the dirt builds up, so that the quality of the printed image is affected.
From the EP-A-0 461 898 there is a printing machine with a cleaning device of the type named above, for which the cleaning roller is a brush roller, which is equipped with helically disposed bristles. The bristles are disposed in several turns on the periphery of the cleaning roller. The cleaning roller is caused to rotate by fictional contact with the rotating printing cylinder, so that the whole surface of the printing cylinder is cleaned uniformly. In one embodiment, the cleaning roller is divided into several axial segments, which can be placed, independently of one another, in contact with the printing cylinder, so that a concerted cleaning of particularly contaminated places on the printing cylinder becomes possible. However, a complicated mechanism is required in order to make or break contact between the individual segments of the cleaning roller and the printing cylinder. In addition, small gaps, in which the surface of the printing cylinder cannot be cleaned, remain between the individuals segments of the cleaning roller.
From the U.S. Pat. No. 5,575,211 there is a cleaning device with a continuous row of bristles, which are stationary at the periphery of the printing cylinder. The cleaning action comes about here owing to the fact that the surface of the rotating printing cylinder is moved past the row of bristles.
In general, cleaning devices are also known, for which the cleaning organs are formed not by bristles but, for example, by section orifices, compressed air nozzles or spraying nozzles for cleaning fluid.
It is an object of the invention to create a printing machine of the type named above, which has a simplified construction and makes possible a simple adjustment to the region of the printing cylinder, which is to be cleaned.
This objective is accomplished owing to the fact that the cleaning organs in every angular position, are disposed on the periphery of the cleaning roller only on a single, axial section of the same and that the cleaning roller can be locked in different angular positions by control equipment.
For the inventive printing machine, an angular position of the cleaning roller is assigned to each axial section of the printing cylinder by a special arrangement of the cleaning organs. In this angular position, the cleaning organs brush exclusively over this section of the printing cylinder, when the peripheral surface of the printing cylinder moves past the cleaning roller. With the help of the control equipment, the axial section of the printing cylinder, which is to be cleaned, can be determined in a simple manner in that the cleaning roller is locked in the corresponding angular position. If an inspection of the printed product reveals that the block must be cleaned in a different axial region of the printing cylinder, the adjustment of the cleaning device can be made in a very simple manner, in that the cleaning roller is twisted somewhat and then locked in the new angular position. Independently of the position of the section that is to be cleaned, the cleaning roller can at all times be in contact as a whole with the printing cylinder, so that the construction of the construction of the mechanism, making or breaking contact, is simplified considerably.
Advantageous developments of the invention arise out of the dependent claims.
Preferably, the cleaning organs are formed by bristles. However, they may also be formed, for example, by cloths, rags, threads or sponge-like material.
Preferably, the cleaning organs are disposed on the periphery of the cleaning roller in the form of a continuous helical line with, at most, one complete turn. A maximum cleaning action is then achieved at the point, at which the helical line faces the surface of the printing cylinder. Since the distance of the helical line to the surface of the printing cylinder and, with that, the contacting force of the cleaning organs, decreases from this point in both axial directions, the cleaning action decreases steadily in both directions, so that there are no sharp transitions between regions of the surface of the printing cylinder, which have and have not been cleaned. Preferably, the helical line has exactly one turn, so that the total angular range of 360xc2x0 of the cleaning roller can be utilized for a precise setting of the axial position, which is effective for the cleaning.
The control equipment for locking the cleaning roller in different angular positions preferably is formed by a stepper motor, so that the position, effective for the cleaning, can be set simply by a command to the stepper motor to shift the cleaning roller into the desired angular position. If a larger axial section of the printing cylinder is to be cleaned, the angular position of the cleaning roller can also be varied during the cleaning operation, so that the position, effective for the cleaning, is xe2x80x9cwobbledxe2x80x9d over an appropriate axial region of the printing cylinder.
In one embodiment of the invention, the position, in which the cleaning roller or brush is in contact with the printing cylinder, is located, in the direction of rotation of the printing cylinder, between the printing position, in which the blocks deliver their ink to the material being printed, and the ink-application position, at which the blocks are inked by an ink-application roller or anilox roller.
In a different embodiment, the cleaning position, in the direction of rotation of the printing cylinder, is located between the ink-application position and the printing position. In this case, the raised portions of the block, which are inked with printing ink, are brushed over by the bristles of the cleaning brush. It was seen that this procedure also does not lead to a noteworthy impairment of printing quality. The action of the cleaning brush consists essentially therein, that the contamination, deposited at the edges of the printing parts of the blocks, is caused to flake off with the help of the bristles of the cleaning brush and is flung away. In this case, the cleaning action is supported owing to the fact that the blocks are moistened by the solvent-containing printing ink.
The mechanism for making and breaking contact between the brush and the printing cylinder, for example, a pneumatic or hydraulic control mechanism, preferably is constructed so that the pressure of the brushes can be adjusted. For example, especially if the blocks are cleaned during the printing operation, it is possible to adjust the brush pressure so that, on the one hand, the desired cleaning effect is achieved and, on the other, the printed image is not affected excessively.